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Abstract

In the first part of this work a volume-limited multiplicity survey with magnitude cutoff (m_K <= 10.5 mag) of 158 young stellar objects located in the Ophiuchus molecular clouds is presented. All companions in the separation range 0.13 arcsec to 6.4 arcsec with flux ratios >= 0.1 at the diffraction limit could be detected by using speckle techniques. The degree of multiplicity is 29.1%+-4.3% and thus only marginally higher than the value 23.5%+-4.8% derived for the given separation range for the main-sequence solar-like stars in the solar neighbourhood. The implications of these findings will be discussed. Afterwards observations with both speckle-cameras and adaptive optics systems are used to improve our knowledge of the dynamical mass of the two companions of the M-dwarf LHS 1070 located in the solar neighbourhood. In the third part observations in the mid-infrared with the interferometric instrument MIDI at the VLTI are described. The reduction of the data with the Fourierspectrum analysis is explained and the interpretation of the results is demonstrated for the binary Z CMa and the triple system T Tau. In the case of Z CMa the position angle, the separation, and the relative brightness of the infrared companion could be determined. From the T Tau data the projected separation between the southern components and their flux ratio is derived. Separate spectra of T Tau N and T Tau S that reveal a silicate absorption towards T Tau S and a spatially resolved spectrum of T Tau N are shown.